FIG. 1 shows a quite common scenario of a communication system SYS for a voice communications infra-structure in corporate networks making use of both a circuit switched telephony network (hereinafter called the voice network) and a V/IP infrastructure composed of H.323 terminals, gateways and possibly controllers (gatekeepers) running over the corporate data network) hereinafter called the multimedia network).
In FIG. 1 the communication system SYS includes in each of a plurality of geographically distant zones A, B, C respectively a zone-specific controller GK-A, GK-B, GK-C and one or more subscriber terminals (endpoints) EP-A1, EP-A2, EP-A3; EP-B1, EP-B2; EP-C1. The subscriber terminals are respectively connected to each other and to the respective controller via a zone-specific bus network A-bus, B-bus, C-bus. A number of routers RA, RB, RC respectively connect the network buses to a first interconnection network INET1. For the first interconnection network INET1 any corporate Wide Area Network WAN, e.g. a corporate multimedia network, a circuit switched network or a packet switched network can be used. Of course, to the private network not only the interconnection network INET1 but also the subscriber terminals, the controllers (gatekeepers) and routers belong which are distributed among a plurality of different geographically distant locations. When the private (multimedia network) is distributed amongst a plurality of locations or zones and is quite extensive, e.g. regarding the numbers of users, each zone ZA, ZB, ZC must include a controller, e.g. so called H.323 gatekeepers, for controlling the bandwidth and the signaling traffic of the network between the subscriber terminals, e.g. the H.323 endpoints, in the network.
For example, as already mentioned above, when the owner of such a private multimedia network or communication system SYS has different subsidiaries in several geographically distant locations (zones), each controller GK-A, GK-B, GK-C is in charge of each zone to hold information about the addresses or calling numbers in such a zone, wherein all these zones are interconnected by means of a corporate WAN, namely the first interconnection network INET1.
Although it is possible that all zones ZA, ZB, ZC belong to the same private multimedia network proprietor, it is also possible that only zones ZA, ZB belong to the private multimedia network of a particular proprietor, whilst one or more other zones ZC belong to yet a different proprietor. Finally, it should be noted that it is also possible that some zones, e.g. the zone ZA, contains subscriber terminals which are capable and have permission to communicate not only within its “own” private network but also within a private network of a further proprietor. All these different signaling procedures, in particular to set up a call between subscriber terminals in the same zone or in two different zones (so-called inter-zone calls) must be handled by the respective controller.
The controller in each zone holds information about all the addresses (calling numbers) of the subscriber terminals, or in the respective zone. More specifically, it holds the prefixes of the E.164 numbers that are assigned to the controller. As shown in FIG. 1, they are interconnected by means of the first interconnection network INET1. Whilst the interconnection network INET1 is sufficient to route calls between the subscriber terminals, there may be a second interconnection network INET2 connected to the respective bus networks A-bus, B-bus via interfaces, e.g. H.323 gateways GW-A, G-B. These telephone gateways convert the signaling on the bus networks, e.g. the H.323 signaling and the media, to whatever transmission standard is used in the voice network, e.g. TDMA, CDMA, etc. Therefore, in principle calls can of course be routed through the second interconnection network INET2 and the gateways GW-A, GW-B if a blocking situation occurs in the first interconnection network INET1.
In FIG. 3a an example is shown for the H.323 standard, i.e. EP-Ai, EP-Bj and EP-Ck are generic H.323 endpoints (subscriber terminals) whereas GW-A and GW-B are telephony gateways. GK-A, GK-B and GK-C are H.323 controllers to control and communicate messages through the different networks INET1, INET2. Since within each location (zone) the controller GK-A, GK-B, GK-C takes care of the signaling traffic between the subscriber terminals, in particular the traffic to set up calls between the individual subscriber terminals belonging to the same private (multimedia) network SYS, generally the controller comprises a message sending means MSM and a message receiving means MRM for sending and receiving messages to/from other controllers (subscriber terminals). Such message sending and message receiving procedures will hereinafter be explained with reference to FIG. 2.
FIG. 2 shows three different examples of message sending/receiving between the individual gate keepers (controllers) when a subscriber station EP-A3 (e.g. a H.323 endpoint) which belongs to the zone ZA controlled by the controller CK-A desires to place a call to another H.323 subscriber terminal EP-B2 in the zone ZB controlled by its corresponding controller GK-B. Conventionally, according to the ITU-TH.323 standard, the controllers in charge of each zone must communicate with other controllers such that the controller GK-A of the calling party EP-H3 knows where to in the private multimedia network the controller GK-A should route the call signaling for such a desired call. Hereinafter, if the call setup relates to the setting-up of the call between different zones, such a call will be called “inter-zone-call”.
FIGS. 2a, 2b, 2c illustrate three different scenarios for routing the “inter-zone call” signaling and they respectively exhibit different disadvantages.
Direct Gatekeeper Routing (DGR)
In FIG. 2a, routing is illustrated which can be directly performed by the controller GK-A when the normal call procedure according to the H.323 standard is followed. In FIG. 2a a subscriber terminal EP-A3 in zone ZA places an admission request message ARQ to its controller GK-A in step ST2a1. In step ST2a2 the controller GK-A returns an admission confirmation message ACF to the subscriber station EP-A3. It should be noted that after step ST2a2 only the principle access or admission request has been granted and no routing numbers have been established or set. In step ST2a3 the subscriber terminal EP-A3 issues a call setup message Setup in order to setup a call to the subscriber station EP-B2. Such a setup message Setup from the subscriber terminal EP-A3 must have normally inserted a calling number of the desired called subscriber terminal, e.g. a E.164 calling number. Although the message receiving means MRM in the controller GK-A clearly recognizes the desired calling number from analyzing the setup message Setup, it still does not actually know where the subscriber terminal EP-B2 is located, e.g. the actual EP-B2 address. The reason for this problem is because within a corporation subscribers may move around from location to location (zone to zone) and the gatekeeper GK-A can do nothing else but guessing from the calling number where the subscriber station is presently located. That is, the gatekeeper GK-A must guess the location of the gatekeeper which is currently serving the desired called subscriber station in order to relay the setup message Setup incoming from the originating endpoint to the correct gatekeeper.
Indirect Gatekeeper Routing (IGR)
Whilst FIG. 2a shows the conventional routing procedure according to the H.323 standard's GK-routed call model, FIG. 2b shows the setup procedure when the H.323 standard's endpoint-routed call model is used. In FIG. 2b the admission request message ARQ in step ST2b1 corresponds to the admission request message ARQ in step ST2a1 in FIG. 2a. However, after step ST2b1, the gatekeeper GK-A controlling the originating zone (GK-A) selects and delivers a so-called transport address (TA) to the calling subscriber terminal EP-A3 in the admission confirmation message ACF. This transport address TA indicates to the originating H.323 subscriber terminal EP-A3 a location to which the originating subscriber terminal EP-A3 should send its call signaling. That is, the transport address indicates (the location of) a gate keeper GK-B which serves the desired called subscriber terminal EP-B2 or in fact the called subscriber terminal itself. Of course, in FIG. 2b the controller GK-A controlling the originating zone GK-A might not know the transport address indicating the location of the destination controller GK-B or GK-C. As is well known and as is shown in FIG. 9b, the_Transport Address (abbreviated TA) is the logical address of an entity located on top of the Transport layer (layer 4) of a network, as defined by the ISO Open Systems Interconnection model.
However, even if the gatekeeper GK-A knew the destination gatekeeper GK-B or the transport address of the gatekeeper GK-B for delivery, in order to eventually reach the called subscriber station EP-B2, the gatekeeper (controller) GK-A controlling the originating zone ZA has no information whatsoever regarding the availability status of the private multimedia network. Therefore, the gatekeeper GK-A cannot guess whether the setup message Setup from the originating subscriber station EP-A3 should be forwarded to the known destination gatekeeper GK-B or alternatively to the originating telephony gateway G-A, such that the call would be routed through the second interconnection network INET2 (the voice networks) and the second telephony gateway GW-B. Such situation may for example occur if the private multimedia network is blocked or breaks down.
Multicast Signaling Procedures (MSP)
As explained above, in FIG. 2a and FIG. 2b there is still some uncertainty as to whether or not the correct terminating controller (gatekeeper) GB-B or its transport address can actually be found. A variation of the H.323 standard, namely the H.323v2 standard of ITU-T, also uses a multicast procedure in order to determine the terminating gatekeeper GK-B which is serving the destination H.323 subscriber terminal EP-B2. Essentially, the multicast procedure is a multiple location inquiry procedure. FIG. 2c shows an example of this procedure.
In step ST2c1 the admission request message ARQ is sent to the controller GK-A which serves the originating subscriber terminal EP-A3. In the admission request message ARQ and alias of EP-B2 is inserted. The “alias” is some kind of identification of the called subscriber terminal EP-B2, i.e. its calling number or another identification. Therefore, “alias” is a generic term for referring to an E.164 number, an e-mail address etc., what is to say any identifier that might be used to find the location of the terminal to which the alias is assigned.
The gatekeeper GK-A sends in step ST2c2 a so-called location request message LRQ to a special Multicast Discovery Transport Address MDTA of all gatekeepers GK-B, GK-C. First, the LRQ message is routed to the corporate network WAN in step ST2c2 and then a plurality of location request messages or location inquiry messages LRQ are respectively sent to the gatekeepers GK-B, GK-C in steps ST2c3, ST2c4. This procedure is called “multicast” since the same location inquiry message LRQ is sent to “multiple” locations. The Multicast Discovery Transport Address is defined in the H.323 standard and is used for the purpose of communication between controllers via “multicast”. Each gatekeeper GK-B, GK-C knows the alias of all H.323 subscriber terminals which are served by the respective gatekeeper. When the received EP-B2 alias and an H.323 subscriber terminal alias match, then a location confirmation message LCF or a location reject message LRJ (if the alias do not match) is returned to the inquiring gatekeeper GK-A in steps ST2c6 and ST2c5.
When the inquiring gatekeeper GK-A receives one location confirmation message LCF from one of the gatekeepers GK-B, GK-C it returns the admission confirmation message ACF to the calling subscriber terminal EP-A3 in step ST2c7. As a result of this, the setup message Setup can be set in step ST2c8 by the calling subscriber terminal EP-A3 and the gatekeeper GK-A can forward the setup message Setup in step ST2c9 to the gatekeeper GK-B which had returned the location confirmation message LCF. As can be seen from FIG. 2c, a large amount of signaling (inquiry and confirmation) messages are necessary in order to determine the gatekeeper according to the H.323v2 standard signaling procedures. In addition, the multicast procedures exhibit quite some significant disadvantages as listed below.
A first disadvantage is, that the identification of the gatekeeper which serves the desired called subscriber station involves the multicast procedure where multiple location request messages must be sent to all gatekeeper locations. Therefore, it may take a considerable time until a location confirmation message LCF is returned and furthermore, the complete procedure may fail depending on the packet load in the interconnection network INET1.
A second disadvantage is that many gatekeepers may be provided in different zones and must receive and answer the multicast messages LRQ, such that additional hardware must be provided in the gatekeepers in order to handle all the LRQ, LCF and LRJ messages, in particular if there are a large number of simultaneous subscriber terminals requesting a call setup.
A third disadvantage is, that the signaling as for example shown in FIG. 2c unavoidably occupies network resources to large extent (flooding the interconnection network INET1 with messages) because at least one multicast packet (LRQ message) must be distributed to all gatekeepers for a single inter-zone call).
A fourth disadvantage is, that unauthorized access to the system can be obtained simply by wire-tapping of the data network INET1 and by sending from a unauthorized gatekeeper unit unauthorized LCF messages as answer to intercepted or listened LRQ messages. Thus, the calling subscriber terminal may communicate with an unauthorized subscriber terminal via an unauthorized gatekeeper without knowing this.
A fifth disadvantage is that it is not possible to setup a virtual private network by assigning one or more H.323 zones to it since any other zone could become part of the virtual private network PN without restriction when the gatekeeper of that zone wanted it (it only has to subscribe to the multicast group).
Thus, the H.323 and H.323v2 standard of ITU-T has a slow call setup, requires additional processing capabilities in the gatekeeper, requires extensive signaling in the network, has a low security and it is impossible to set up a virtual private network.